Kinematic calibration and investigation of the influence of universal joint errors on accuracy improvement for a 3-DOF parallel manipulator

- The error model of the 3-DOF parallel manipulator is established by D-H based methods such that all of the potential error parameters can be considered. This error modeling method can also be used for most of the non-overconstrained parallel manipulators.
- The kinematic parameters of the 3 limbs can be identified simultaneously based on the proposed calibration model.
- The influence of the defects of universal joints on accuracy improvement after kinematic calibration is investigated.

This paper focuses on the accuracy enhancement of a 3-PRRU parallel manipulator and the influence of imperfect universal joints on calibration result through kinematic calibration. The error model of the PRRU limb that contains all of the identifiable kinematic errors is established based on modified Denavit-Hartenberg method and Hayati convention. By eliminating the motion errors of passive joints and combining all of the limbs' error models, the calibration model of the parallel manipulator is obtained. To verify the effectiveness of the error model and investigate the influence of the imperfect universal joints on positioning accuracy improvement, numerical simulations and experiments of kinematic calibration are respectively conducted and total least squares method is used to identify the kinematic parameters. The results of the simulations and experiments show that the defects of universal joints have smaller effect on the accuracy improvement for this manipulator when compared with the cases that the universal joints are perfectly manufactured and assembled.